Development of Superconducting Devices for Power Grids in Italy: Update About the SFCL Project and Launching of the Research Activity on HTS Cables

1. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017 5600406
Development of Superconducting Devices for Power
Grids in Italy: Update About the SFCL Project and
Launching of the Research Activity on HTS Cables
G. Angeli, M. Bocchi, M. Ascade, V. Rossi, A. Valzasina, and L. Martini
(Invited Paper)
Abstract—In the framework of the development of High Temper-
ature Superconducting (HTS)-based solutions for modern power
grids, Ricerca sul Sistema Energetico - RSE S.p.A. has gained
a significant experience in design, testing, and installation of su-
perconducting fault current limiters (SFCLs). After having suc-
cessfully demonstrated the techno-economic feasibility of the first
Italian three-phase BSCCO-based SFCL (9 kV/3.4 MVA) inside
the distribution grid of A2A Reti Elettriche S.p.A. in the Milano
area, RSE has initiated the development of a new upgraded three-
phase SFCL, whose rated power has been enhanced in order to
meet the utility’s needs. Unlike the first prototype, the new SFCL
device (9 kV/15.6 MVA) is YBCO-based: this choice allows reduc-
ing the needed tape length and cooling power, saving at the same
time a significant amount of operational costs. The first phase was
already assembled and tested in 2015, whereas the second phase
was tested in February 2016. This paper will show the last out-
comes from characterizations and short-circuit tests. Along with
the SFCL project, RSE has also been focusing on superconducting
cables and, in the framework of the European Project Best Paths,
it has been involved in DEMO5, that is dedicated to the devel-
opment of a HTS DC cable in MgB2 (10 kA/3.2GW). Taking the
cue from the DEMO5 requirements, RSE has initiated a parallel
research activity that is focused on studying the thermo-fluid dy-
namic behavior of cryogenic coolants flowing in forced convection
regime inside the flexible cryostat. This paper will present the first
simulation results and will give some anticipation on next steps.
Index Terms—SFCL, distribution grid, short-circuit current, in-
field activity, HTS cable, cryogenic coolant.
I. INTRODUCTION
THE energy system of most countries worldwide are un-
dergoing a very rapid evolution. The reasons for these
changes are found in several trends, in particular the increas-
ing consciousness of the importance of reducing greenhouse
gas emissions and mitigating the effects of climate change, the
Manuscript received September 6, 2016; accepted October 24, 2016. Date of
publication December 13, 2016; date of current version January 24, 2017. This
work was supported by the Research Fund for the Italian Electrical System under
the Contract Agreement between RSE S.p.A. and the Ministry of Economic
Development in compliance with the Decree of March 8, 2006.
The authors are with the T&D Technologies Department, Ricerca sul Sistema
Energetico—RSE S.p.A, Milan 20134, Italy (e-mail: giuliano.angeli@rse-web.
it; Marco.Bocchi@rse-web.it; massimo.ascade@rse-web.it; valerio.rossi@
rse-web.it; angelo.valzasina@rse-web.it; luciano.martini@rse-web.it).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2016.2639022
necessity of adopting all means of increasing energy efficiency
and reducing energy consumption, the geopolitical events af-
fecting countries mutual relationships, the progressive aging of
the electricity infrastructures that need to be modernized by
deploying new technologies to enable upgraded capabilities,
advanced real-time monitoring, better failure detection, and im-
proved cyber-security [1].
It is also to be considered that the progressive decarbonisation
of European grid implies new technological challenges in order
to meet the commitments taken by Member States for 2020
and beyond. In particular renewable energies are going to play
an important role to contribute to greenhouse gases emission
abatement by 2050.
High Temperature Superconducting (HTS) devices may be
one of the key elements to facilitate an energy-efficient and cost-
effective transition towards modern power grids [2]. One of the
key factors for HTS devices to compete with traditional equip-
ment is the optimization of simulation and design methodologies
in order to maximize their performances at reduced costs.
Nevertheless the development of tools aimed at simulating the
consequences of fault events is essential for meeting the needs
of real grids. The working condition of any HTS device, that is
cooled by means of a cryogenic coolant, may be significantly
affected by the energy injected inside the coolant during a fault
event: temperature and pressure may dramatically increase and
even changes in the coolant physical state (from liquid to gas)
may occur.
In the following, development and testing of Superconduct-
ing Fault Current Limiters (SFCL) and preliminary study about
superconducting (SC) cables will be addressed.
SFCL devices could allow utilities to avoid or delay the up-
grading of existing power equipment (e.g. transformers, busbars,
lines, protections) to effectively handle increasingly higher elec-
trical surges. Moreover, the SFCL may improve power quality
by allowing interconnection of grids and by enhancing the sta-
bility of the grid [1], [3], [4]. HTS cables are quite attractive for
their low transmission losses and high power density, making
this application an interesting solution to satisfy the typically in-
creasing electricity demand in highly populated urban areas [5].
The self-shielding of electromagnetic field and the use of
environmentally friendly coolant are additional benefits of using
HTS cable instead of copper cable [6].
1051-8223 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications standards/publications/rights/index.html for more information.

2. 5600406 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017
II. THE NEW UPGRADED SFCL FOR THE ITALIAN PROJECT
A. Introduction
The results coming from the experimentation on the BSCCO-
based SFCL device allowed demonstrating the technical feasi-
bility of integrating such a device in the Milano live-grid be-
longing to A2A Reti Elettriche S.p.A. (A2A). The highest point
of the experimentation was the three phase fault test organized
by RSE and A2A in May 2014 [7].
On the basis of the first stage results, it was agreed to launch
the second stage, that is devoted to ascertain the device behavior
in an installation site considered critical by A2A because of high
short-circuit currents. For reaching this goal, the design of an up-
graded SFCL device has been initiated with the new SFCL pro-
totype to be installed in the same substation but as a transformer
protection, therefore protecting four different feeders, with a
higher probability of fault occurrence. This new challenging
goal implies an enhancement in rated power, up to 15.6 MVA,
and a different layout based on 2G HTS tapes. The choice of
adopting the YBCO-based design has been motivated by the im-
proved characteristics featured by last-generation YBCO tapes,
allowing significant reduction of devices realization and opera-
tional costs (in particular the needed refrigeration power and the
total HTS amount). Making a comparison with BSCCO tapes at
the same temperature and current conditions, YBCO tapes fea-
ture critical current up to 1.5 ÷ 2.5 times higher, normal state
resistance about 6 ÷ 10 times larger and AC losses 5 ÷ 10 times
lower [8]. In 2015 the first phase of the upgraded 9 kV/ 1 kA
SFCL device was successfully short-circuit tested and it was
made of 2 coaxially-arranged parallel-connected YBCO-based
coils. Each coil was manufactured by means of a HTS tape
amount of 96 m anti-inductively wound on a fiberglass cylinder.
The short-circuit tests were performed at the CESI test facility
in Milano and the device limited the first prospective peak of
14.8 kA down to 10.4 kA, featuring a Limitation Factor (LF)
value of 1.4. In 2016 similar tests were performed to validate the
second phase and the main results are presented in the following.
B. YBCO-Based Coils Characterizations
In order to finalize the design of the complete upgraded de-
vice, several characterizations were carried-out to measure the
critical current at 65 K and 77 K and the AC losses level for
different injected current peaks. The final goal was identifying
and validating the two HTS coils that would have been used as
the second phase of the upgraded device.
The YBCO windings underwent experimental DC measure-
ments for the evaluation of the critical current in a conservative
way by considering 0.05 μV/ cm as electric field threshold. The
procedure consists in measuring the current and voltage drop
across each HTS winding immersed in a liquid nitrogen bath
and the current injection is performed by a series of steps last-
ing 1 s and with increasing amplitude. Fig. 1 and Fig. 2 refers
to a YBCO-based single-coil prototype measuring 95.8 m in
length, that has been tested in both DC (at 77 K and 65 K)
and AC (at 50 Hz and four different temperatures in the tem-
perature interval 65 K–77 K). The single-coil prototype was
Fig. 1. Electric field as a function of injected current on the YBCO-based
single-coil preliminary prototype at temperatures of 65 K and 77 K.
Fig. 2. AC losses as a function of the AC current injected peak for the same
YBCO-based preliminary prototype at 50 Hz and at four different temperatures
(65.3 K, 69 K, 73.2 K, 77.3 K).
TABLE I
SUMMARY OF THE DC TESTS ON THE 95 M-LONG YBCO COIL PROTOTYPE
T (K) Rconn (nΩ) Ic ( 0.05 μV/cm) (A) Ic ( 0.1 μV/cm) (A) N
65 178 1042 1060 39
77 264 447 456 36
made of two overlapped 12 mm-wide YBCO tapes wound on
a fiberglass support (external diameter of 246 mm) by means
of 61 turns for each layer. At 77 K, the critical current Ic mea-
sured at Ec = 0.05 μV/cm was 447 A: critical current at higher
electric fields was numerically estimated and resulted 456 A at
0.1 μV/ cm with an exponent N (see EHT S model formula in
Fig. 1) of about 36. At 65 K, the critical current Ic measured
at 0.05 μV/cm was 1042 A, whereas it has resulted 1060 A at
0.1 μV/ cm and the exponent N was equal to 39.
Table I reports a summary of the DC test results. The term
Rconn includes the resistance of the connections between the
HTS coil prototype and the external current leads and also the
resistance due to the joint between the two anti-inductive wind-
ings. Even if this contribution is in the order of few hundreds nΩ,
the final effect is significant, as Fig. 1 shows: Emeas is the volt-
age drop measured during tests, including all the contributions,

3. ANGELI et al.: DEVELOPMENT OF SUPERCONDUCTING DEVICES FOR POWER GRIDS IN ITALY: UPDATE ABOUT THE SFCL PROJECT 5600406
whereas EHT S represents the voltage drop without all resistive
contributions not directly depending on the HTS wire. The first
part of Emeas features a linear behavior since the supercon-
ductor has not started any transition yet and the prevalent contri-
bution is due purely to Rconn. As soon as the superconductor
starts the transition, its resistive voltage drop becomes quickly
higher than any other contribution, therefore the waveform as-
sume the typical superconducting exponential behavior, with a
steepness depending on the exponent N. Numerical elaborations
were also carried-out to calculate the parameters of the fitting
functions that better represent the behavior of HTS tape. In par-
ticular, when the model of electric field behavior (EHT S model)
as a function of injected current has been considered, the model
functions calculated on the basis of experimental measurements
by means of best fitting techniques, allowed to evaluate also the
electric field values exceeding the measurements threshold, that
was conservatively low (typically 0.05 μV/cm).
AC measurements were performed by inserting HTS winding
in a liquid nitrogen bath inside a vacuum insulated cryostat. The
testing methodology is based on the injection of AC currents at
50 Hz with increasing amplitude. The sampling frequency used
for the acquisition of the voltage across the coil and of the cur-
rent is 100 kHz, whereas the current injection duration is about
200 ms (10 periods). Experimental measurements are elaborated
by means of a software that allows performing graphical routines
and numerical calculations. Measurements were carried-out at
four different temperatures (77.3 K, 73.2 K, 69 K, 65.3 K) and
Fig. 2 shows the behavior of the AC losses for each temperature
as a function of the peak injected current.
As shown in Fig. 2, AC losses are temperature-dependent,
since their actual value is a function of the ratio between the
injected current peak and the critical current. Considering the
nominal current of 1000 Arms (1414 A peak value) and that each
electrical phase is made of two windings parallel connected, we
can assume that each winding carries 50% (i.e. 727 A) of the
whole current. From Fig. 2, 77 K is not the proper working
temperature, since AC losses level is too high; whereas, at 69 K
and 65 K the AC losses level is around 80 mW/m and 55 mW/ m
respectively, corresponding to 48 W and 33 W over the 600 m
total amount of HTS tape needed to develop the three-phase
SFCL prototype.
C. Short-Circuit Test on Single-Phase 9 kV/ 1 kA SFCL
The prototype that underwent short-circuit tests constitutes
the second phase of the final three-phase prototype and may be
considered as a full scale single-phase SFCL. It is made of two
parallel-connected YBCO windings, each composed by a total
HTS tape amount of 96 m antinductively wound by means of
two layers properly superimposed to make the self-field almost
negligible. The prototype was immersed in a liquid nitrogen
bath at 65 K.
Following a conservative approach, it was decided to perform
two types of tests, the first at reduced prospective current and
the second with the full prospective current. The measurements
acquired were: the total phase current, current flowing through
the HTS winding, phase voltage upstream shunt reactor, phase
voltage downstream shunt reactor and the voltage drop across
TABLE II
RESULTS OF SHORT-CIRCUIT CALIBRATION TESTS WITHOUT SFCL
Prospective current Duration Prospective fault Short circuit
current amplitude (Ar m s ) (ms) 1° peaks value (A) power factor
6022 80 15090 0.06
10920 80 26920 0.06
Fig. 3. Comparison between prospective (IPSC ), limited (ILIM ) and HTS
current during the reduced short-circuit current test (test 1, T = 65 K,
IPSC rm s = 6 kArm s ).
the HTS winding. The latter is the most critical one, since it
is negligible during the nominal operating conditions, but it
assumes very high values as soon as a short-circuit occurs. The
data sample frequency was 20 kHz.
Preliminary numerical simulations were carried-on before
power tests, in order to estimate the electro-magnetic and ther-
mal stress that the device were going to undergo and for properly
setting the testing circuit parameters, including the measure-
ment systems. Further to the numerical simulations, tests were
performed at CESI laboratories (Milano) after having properly
set the test facility parameters. Table II summarizes the main
parameters recorded during the calibration tests on the circuit
without SFCL. These tests allow to identify the circuit param-
eters to be set for carrying-out the two short-circuit tests with
the SFCL included in the circuit (Test 1 identifies the reduced
current test, whereas Test 2 the full current test). Fig. 3 shows
the comparison between the prospective (IPSC ) and the limited
(ILIM ) short-circuit current during reduced current test (Test 1).
The reduced short-circuit current test was performed at 65 K and
applying 4.95 kVrms. The tested device was shunted by a reactor
with reactance value Xreactor of 0.367 Ω and resistance value
Rreactor of 0.024Ω. The fault inceptions is preceded by a nom-
inal working conditions lasting 270 ms and involving a current
of 764 Arms. As soon as the fault occurs, the first prospective
peak of 15 kAp is limited to 10.5 kAp, therefore implying a LF
of 1.44. It is worthwhile also emphasizing how short-circuit
current is divided between HTS windings and shunt reactors:
considering the first peak, about 3.5 kA out of the total 10.5 kA
flows through the HTS windings (IHTS), whereas the remaining
part is shifted towards shunt reactors.
Table III reports the values of the first six peaks of IPSC , ILIM ,
IHTS, along with the corresponding LF achieved.

4. 5600406 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017
TABLE III
LIMITATION FACTORS ACHIEVED DURING THE REDUCED
SHORT-CIRCUIT CURRENT TEST FOR THE FIRST 6 PEAKS
Peak Number IP S C (kAp ) IL IM (kAp ) IH T S (kAp ) LF
1st
15.09 10.49 3.63 1.44
2nd
−2.83 −1.95 −0.69 1.45
3rd
12.86 8.99 0.58 1.43
4th
−4.66 −3.16 −0.52 1.47
5th
11.39 7.95 0.48 1.43
6th
−5.84 −3.97 −0.45 1.47
Fig. 4. Comparison between prospective (IPSC ) and limited (ILim ) current
during the full short-circuit test (T = 65 K, IPSCrm s = 11kArm s ).
TABLE IV
LIMITATION FACTORS ACHIEVED DURING THE FULL SHORT-CIRCUIT
CURRENT TEST FOR THE FIRST 6 PEAKS
Peak Number IP S C (kAp ) IL im (kAp ) IH T S (kAp ) LF
1st
26.92 15.16 3.67 1.78
2nd
−6.27 −3.09 −0.77 2.03
3rd
22.16 12.74 0.64 1.74
4th
−9.88 −4.96 −0.58 1.99
5th
19.40 11.19 0.53 1.73
6th
−1.99 −6.17 −0.50 1.94
Alike the previous test, also the full short-circuit current test
was performed at 65 K and applying 4.95 kVrms to the test
circuit. The tested device was shunted by a reactor with reac-
tance value Xreactor of 0.367 Ω and resistance value Rreactor of
0.024 Ω. Fig. 4 shows the comparison between IPSC and ILim
during the full short-circuit current test. Limiting the first peak
current from 26.92 kAp to 15.16 kAp, the resulting LF achieved
is 1.78. The first peak of the current IHTS flowing through the
HTS windings is 3.67 kAp, therefore about 20% of the total
limited current. Table IV shows the values of the first 6 peaks
of IPSC , ILim , IHTS, along with the corresponding LF achieved.
Fig. 5 compares the behavior of the short-circuit current flowing
through each HTS winding and the increase in HTS resistance
up to about 16 Ω and 13 Ω respectively. The gap between the
two values is due to different final temperatures that may be
caused by small differences either in the HTS wire or in the two
windings layout.
Fig. 5. Short-circuit current in the two HTS windings and time-behavior of
the corresponding resistances.
Fig. 6. Comparison between dissipated power and temperature in the two
HTS windings composing the phase under the full short-circuit current test.
The temperature gap between the two HTS windings may be
appreciated in Fig. 6, that compares the temperature behavior
to the power dissipated by each winding. The final temperature
gap turns out to be about 20 K (being respectively 222 K and
201 K).
III. THERMO-FLUID DYNAMIC MODELING FOR HTS CABLES
This activity has taken the cue from the RSE involvement in
the European Project BEST PATHS (“BEyond State-of-the-art
Technologies for rePowering Ac corridors and multi-Terminal
HVDC Systems”), whose DEMO5 is focused on the develop-
ment of a HVDC MgB2 cable with 3.2 GW power capability
and cooled by means of a two systems in parallel (in helium at
20 K and in liquid nitrogen at 77 K).
One of the main objectives of this activity is developing math-
ematical models for the simulation of the thermo-fluid dynamic
behavior of cryogenic fluids flowing in forced convection inside
the cryostat of superconducting (SC) cables. The final goal is
to develop useful tools for assessing the behavior of the cable
stability under varying conditions that may affect the effective-
ness of its refrigeration, the cable electromagnetic behavior and
consequently its safety and reliability.
In this work, starting from the fundamental equations of fluid
dynamics, a mathematical model has been developed that repre-
sents a monodimensional formulation in steady-state conditions
of thermo-fluid-dynamic behavior of cryogenic fluids flowing

5. ANGELI et al.: DEVELOPMENT OF SUPERCONDUCTING DEVICES FOR POWER GRIDS IN ITALY: UPDATE ABOUT THE SFCL PROJECT 5600406
in forced convection within the typical tubular cryostats used
for the refrigeration of SC cables. The basic hypotheses for the
forthcoming study are as follows:
1) the cryostat cross-section is constant and the SC cable
prevalent component is along the x-axis and any different
dimension (e.g. cable diameter) is negligible if compared
with length;
2) thermodynamic properties (e.g. temperature, enthalpy, en-
tropy, internal energy, density, specific heat) of the cryo-
genic coolant can vary along the x-axis, but are constant
in other directions;
3) thermodynamic and thermo-physic properties are in
steady-state regime, therefore any time-variation is pre-
vented, moreover the cryogenic coolant does not undergo
any phase transition (e.g. from liquid to gas) while flowing
through the cryostat;
4) the flow velocity is monodimensional, therefore its com-
ponent vx along x-axis is the only one to be considered
v = vx
5) the cryogenic coolant may be subject to an internal spe-
cific heat power generation qGE N int
, due to the SC cable
dissipation, that can be dependent on the position along
the x-axis;
6) the cryogenic coolant may receive the heat power qHE ext
from the external environment by means of convection,
conduction and radiation (this heat power contribution
can be dependent on the position along the x-axis);
7) the conduction heat transfer between adjacent fluid vol-
ume elements is supposed to be negligible with respect to
contributions coming from mass flow, internal heat gen-
eration and external thermal input.
The physical model consists in a cylindrical cryostat with an
inner cylinder representing the SC cable. The cryogenic coolant
is flowing in the space comprised between the two cylinders.
The mathematical model is aimed at evaluating the spatial dis-
tribution of relevant thermodynamic properties of the coolant.
A dedicated software has been developed, that is used to select
the type of cryogenic coolant flowing in forced flow convec-
tion regime inside the cryostat, therefore it can be adapted to
any coolant. The thermodynamic and thermo-physic properties
of coolants, as functions of temperature and density, are repre-
sented by mathematical functions embedded inside the software.
These functions feature the same analytical formulation, but they
have different coefficients depending on the coolant. The initial
choice of the coolant allows the software to automatically im-
plement the algorithm on the basis of the coefficients related
to the selected coolant. The thermodynamic and thermo-physic
properties of coolants have been implemented using the formu-
lations reported in [9], [12]. As a results, the aforesaid approach
led to the following non-linear system of ordinary differential
equations:
F11 ( T , ρ ) ·
∂ T
∂ x
+ F12 ( T , ρ ) ·
∂ ρ
∂ x
+ G1 ( T , ρ ) = 0 (1)
F21 ( T , ρ ) ·
∂ T
∂ x
+ F22 ( T , ρ ) ·
∂ ρ
∂ x
+ G2 ( T , ρ ) = 0 (2)
where T and ρ are respectively temperature and density of the
coolant. Eq. (1) and (2) are obtained with some manipulations
from the equations of energy and momentum conservation for
unit length and unit volume, whereas the equation of mass con-
servation, under the hypothesis of stationary regime of the fluid
monodimensional motion, states that the specific mass flow rate
λ = ρ · v, where v is the fluid velocity, is constant with respect
to time and space. The variables T, ρ and v are functions of x, i.e.
the position along the cryostat axis; the coefficients Fij and Gi
with i, j = 1,2 in (1) and (2) are analytically defined as follows:
⎡
⎢
⎢
⎣
F11 =
λ
ρ
·
∂ p
∂ T
− ρ ·
∂ h
∂ T
F12 =
λ
ρ
·
∂ p
∂ ρ
− ρ ·
∂ h
∂ ρ
F21 =−
λ
ρ
·
∂ p
∂ T
F22 =
λ
ρ
·
λ2
ρ2
−
∂ p
∂ ρ
⎤
⎥
⎥
⎦(3)
G1=
qGE N i n t
+
+qHE e x t
=qT O T
; G2=−λ·g· sin α−
λ
ρ
·
Pfl
Afl
·τfl
(4)
In (3) the partial derivatives of pressure p and specific enthalpy
h with respect to T and ρ are used: they have been calculated
from functions p(T, ρ) and h(T, ρ), whose analytical description,
embedded in the mathematical model, are the same reported in
[9] for each cryogenic coolant. In (4) g is the gravity accelera-
tion, α is the slope of the cryostat, μ is the fluid viscosity (that
is function of T and p in agreement with [12]), Pfl
and Afl
are
respectively the perimeter and cross-section of the area involved
in the flowing of coolant, τfl
is the shear stress due to the friction
between the fluid and the cryostat. τfl
is given by the following
equation:

6. τfl
= fK at h
(Re, Φ) ·
λ2
2 · ρ
; Re =
λ · Dh
μ
; Dh = 4 ·
Afl
Pfl
(5)
In (5) fK ath
is the Fanning friction factor, that is defined by
the Katheder correlation described in [10], [11] as function of
the Reynolds number Re and of the cross-section void-fraction
Φ = Afl
/Aduct
(where Aduct
is the whole cryostat cross-section).
The numerical simulation software has been used to simulate
the Liquid Nitrogen (LN2) flowing in forced convection through
the SC cables cryostat, that is geometrically modeled as a cylin-
drical conduit containing another cylinder representing the SC
cable. The software has been developed in order to calculate the
spatial distribution of all the LN2 physical and thermodynamic
characteristics along the conduit.
In the following, all the calculations are concerned with a
cryostat featuring the geometrical dimensions reported below:
r Dext = External diameter of LN2 annulus pipe = 0.12 m
r Dint = Internal diameter of LN2 annulus pipe = 0.09 m
r L = Cable length = 500 m
and the following heat load has been taken into consideration:
r qT OT = Heat load in cryogenic coolant = 5 W/ m
LN2 in the inlet section is characterized as follows:
r T (x = 0) = LN2 inlet temperature = 75 K
r p (x = 0) = LN2 inlet pressure = 2 bar
the calculation has been made with the following mass flow
values [13], [14]:
r Q = Mass flow between 0.1 kg/s and 1.0 kg/ s step 0.1 kg/ s

7. 5600406 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017
Fig. 7. LN2 temperature and pressure variations along the axial position
parametrized on the mass flow rate values respect the inlet section.
Fig. 8. GHe temperature and pressure variations with respect to the inlet
section along the axial position parameterized on the mass flow rate.
where Q = λ · Afl
and it consists in a series of 10 simulations.
Results are shown in Fig. 7, that reports the variation of pressure
and temperature [15], calculated with respect to the inlet section,
along the conduit axis parametrized on the mass flow rate values.
As an example case study, further simulations have been per-
formed assuming the usage of a different coolant, i.e., gaseous
Helium, for the same SC cable refrigeration. GHe in the inlet
section is characterized as follows:
r T (x = 0) = GHe inlet temperature = 50 K
r p (x = 0) = GHe inlet pressure = 5 bar
the calculation has been made with the following mass flow
values:
r Q = Mass flow between 30 g/ s and 84 g/ s with step 6 g/ s
and it consists in a series of 10 simulations. Results are shown in
Fig. 8, that, alike Fig. 7 for the LN2 coolant, reports the variation
of pressure and temperature, calculated with respect to the inlet
section, along the conduit axis parameterized on the mass flow
rate values.
IV. CONCLUSION
The second step of the Italian project for the installation
of a three-phase SFCL device in the distribution grid of Mi-
lano has been launched: it foresees the development of the
upgraded device 9 kV/15.6 MVA. The first phase was success-
fully short-circuit tested in first quarter of 2015, whereas the
second phase has passed the tests in 2016. This paper has sum-
marized the main outcomes from the second phase short-circuit
testing and the numerical elaborations based on the gathered
experimental data has been discussed. Along with the SFCL
project, RSE has recently launched a new research activity fo-
cused on the study of thermo-fluid dynamic behaviour of cryo-
genic coolant flowing in forced convection through the super-
conducting cables cryostat. A dedicated software has been de-
veloped, that includes all the properties of cryogenic coolant
expressed as functions of density and temperature.
The preliminary numerical simulations implementing a sta-
tionary monodimensional model of superconducting cable
cooled down by means of nitrogen and helium has been pre-
sented and discussed. The next steps will consist in developing
a non-stationary model including also the coolant phase transi-
tion (liquid to gas) along the cryostat.
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